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Saturday, December 6, 2014

The Phenomenology of Peripheral Vision

How narrow is your foveal center--the part of your vision with complete clarity? How precipitously does that clarity fade into the periphery? Where exactly does vision end completely?

Imagine you had a hula-hoop about the diameter of your wingspan, so that you could hold it up to make a circle around your head on a plane with your eyes. Now imagine there are random numbers on the hula hoop spaced about an inch apart. How many of those numbers do you imagine you'd be able to make out at once?

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(The following experiment is adapted from (second-hand discussions of) one described in Dan Dennett's Consciousness Explained.)

If you have a regular deck of playing cards, take that out and skip the next paragraph.

If you don't have a deck of playing cards, get some index cards and a pen. In a pinch, you can just cut up some blank paper. You'll need ten cards. Write the numbers zero through nine, one each, in the center of the cards. Make the numbers of consistent size, about as big as your thumbnail.

Shuffle the deck. Draw a card and hold it at arms length in front of you with your left hand, so the number is facing you. If you can't make it out clearly at arm's length, move it toward you until it's in focus.

Then, keeping your gaze fixed on the first card, draw another card with your right hand, and hold it at arm's length (or however far away the first card is), but out to the right.

Move your right hand back behind you until the card disappears from your vision. Move it in and out of your vision, a hair at a time, until you're sure exactly where it disappears. Does it feel completely binary--now you see the card, now you don't--or is there a point where you'r not completely sure whether you can see the card or not?

Very slowly, not letting your gaze waver from the first card, move your right hand in toward your left, inch by inch.

There may be a point where you feel about 50% sure you have correctly identified the card in your right hand, but you can't make it out clearly. Note about how many inches apart the centers of each card are at that point. If you've got a friend around to help, you can even have them measure for you.

Keep moving your right hand toward your left until you feel certain you've correctly identified the second card. How many inches apart are the cards now?

You might even try moving the cards even closer until the number in your right hand is just as clear as the number in your left hand.

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I find that I can't actually do this last part without folding one of the cards, because the numbers must be touching. Even then I notice that I can shift my gaze slightly to bring the number on the right into even sharper focus.

A lot of people are surprised by this experiment. Many think there's equally high clarity for about 30 degrees of arc, and update on the results of this experiment to two degrees of arc. They learn that their foveal vision is much narrower than they thought.

Ponder the implications of that for a minute.

People are wrong about what it's like to have peripheral vision. Peripheral vision is, presumably, part of every sighted person's experience for many hours a day. Yet, you ask them questions about their ongoing subjective experience of vision while their eyes are open, and they report falsehoods.

Maybe the experiment itself modifies peripheral vision, rendering the foveal center artificially narrow, and people are actually correct in their beliefs about vision the whole time. My priors are pretty strongly against that, though, and I think my own account of what's going on is stronger.